The present invention relates to an economical process and apparatus for effectively and uniformly removing oxides from the surface of a hot workpiece by applying a uniform liquid force against the surfaces of the workpiece as it travels through a rolling mill train. More particularly, it relates to descaling hot steel slabs, plate or strip in hot mill rolling operations, especially where descaling stations are located in the roughing and finishing stages of the rolling operations.
Conventional practices for descaling or removing oxides from a workpiece in a hot strip rolling mill where the workpiece is passed through the several stands of the mill carrying work rolls for receiving the workpiece for its reduction generally involves the use of nozzles. Typically, several nozzle header arrangements are positioned adjacent to and between the stands for spraying water under high pressures of 2000 to 3000 psi across the top and bottom surfaces of the workpiece basically to remove the scale formations as taught in U.S. Pat. No. 3,518,736.
These nozzles of these header arrangements are designed such that they produce an elongated elliptical impact zone and are usually angled and canted between 10.degree. to 15.degree. resulting in an overlapping spray effect when the sprays impinge against the surface of the strip as shown in the drawings herein. The sprays of these angled and canted nozzles when tested on surfaces such as pressboard or soft lead create obvious impact impressions which vary from shallow to deep. Such impact patterns mean that similar patterns are produced on a surface of a hot steel workpiece for scale removal in that the surface temperature varies along the elliptical impact zones of the spray nozzles.
In addition to these impact pressure variations, there exists water volume variations at different locations within the elliptical impact zones which also create this cyclic temperature pattern across the width of the workpiece. This cyclic temperature pattern of the present descaling nozzles can actually be seen as a longitudinal streaking pattern on the surface of the workpiece, particularly upon its travel from the roughing mill to the finishing train. Such streaking patterns indicate erratic scale removal by the present descaling systems and it is apparent that these streaking patterns are detrimental to both the quality of the workpiece and the rolling operations.
Regardless of the location of the nozzles relative to the workpiece, it is inherent due to the nozzle design, that cyclic pressure impact patterns are always produced resulting in cyclic scale removal as well as cyclic surface temperature patterns across the width of the workpiece. This cyclic temperature pattern results in cyclic secondary scale formation and cyclic rolling characteristics in the workpiece upon subsequent rolling operations.
As mentioned above, this uneven or cyclic removal of scale is unsatisfactory in that the end result is poor surface quality and non-uniform metallurgical characteristics in the finished product. Also, as the workpiece travels from one stand to the next, the newly formed secondary oxides on the workpiece are continually forced against the surface of the work rolls eventually causing non-uniform wear patterns thereon, which rolls then have to be reground or changed much more frequently compared to the condition where there is uniform water volume, uniform pressure and uniform temperature distribution across the surface of the workpiece.
The pressure of the liquid delivered to the present nozzle descaling arrangements fall in an extremely high range demanding very costly and complicated descaling equipment.